1. Field of the Invention
The present invention relates to printing apparatus such as electrophotographic copying machines and electrophotographic printers in which images are printed by electrophotography.
2. Description of the Related Art
A conventional printing apparatus of this type performs printing through a sequence of electrophotographic processes: charging, exposing, developing, transferring, and cleaning. The apparatus incorporates a print engine that takes the form of a unit assembly and performs printing operations.
For performing the aforementioned sequence of photographic operations, the print engine receives high negative voltages and high positive voltages from power sources provided in the main body of the printing apparatus. A charging roller receives a high negative voltage and other rollers including a developing roller receive low negative voltages.
The printing mechanism is removably attached to the main body. The printing mechanism and the main body have mechanical contacts such that when the printing mechanism is attached to the main body, the printing mechanism receives negative voltages through the mechanical contacts.
The contacts provided on the printing mechanism are, for example, electrically conductive plate-like members that are in contact with the end surface of an electrically conductive shaft of the respective rollers. The contacts provided on the main body are, for example, electrically conductive resilient plate-like members.
The aforementioned conventional printer suffers the following problems.
If the printing mechanism is repeatedly mounted on and dismounted from the main body over a long term use, the electrical contact members will be subjected to mechanical deformation and fatigue, or attract foreign materials such as toner particles and paper particles. Mechanical fatigue causes the contact members to lose resiliency.
As a result, even if the printing mechanism is attached properly into the main body, the electrical contact may not be properly made between the printing mechanism and the main body. Poor electrical contact causes problems such as improper sequence of electrophotographic processes and poor print quality. Prolonged poor electrical contact eventually leads to damages to the print engine, particularly to electrical and physical properties of the photoconductive drum. The present invention was made in view of the aforementioned problems.
An electrophotographic printing apparatus includes at least one printing mechanism that prints on a print medium transported on a transport belt to the printing mechanism. The printing mechanism is movable between an operative position and a non-operative position thereof. The printing mechanism and the main body have mechanical contacts through which electric power is supplied from the main body to the printing mechanism when the printing mechanism is at the operative position. The apparatus also includes a density sensor and a controller. The density sensor is located downstream of the printing mechanism with respect to a direction of travel of the transport belt. The density sensor detects the density of a surface of the transport belt when a test pattern is printed on the transport belt. The controller causes the printing mechanism to print the test pattern on the transport belt. Then, based on the output of the density sensor, the controller performs a comparison operation in which a density of the printed test pattern is compared with a density of print data of the test pattern. When the controller detects a discrepancy in density between the data of test pattern and the printed test pattern during the comparison operation, the controller generates a reporting signal indicative of the discrepancy.
The controller may perform the comparison operation a certain number of times when the controller detects the discrepancy.
When the controller does not detect the discrepancy during the comparison operation, the controller enters a standby state where the printer waits for a print job.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.